Information retrieval systems, generally called search engines, are now an essential tool for finding information in large scale, diverse, and growing corpuses such as the Internet. Generally, search engines create an index that relates documents (or “pages”) to the individual words present in each document. A document is retrieved in response to a query containing a number of query terms, typically based on having some number of query terms present in the document. The retrieved documents are then ranked according to other statistical measures, such as frequency of occurrence of the query terms, host domain, link analysis, and the like. The retrieved documents are then presented to the user, typically in their ranked order, and without any further grouping or imposed hierarchy. In some cases, a selected portion of a text of a document is presented to provide the user with a glimpse of the document's content.
Direct “Boolean” matching of query terms has well known limitations, and in particular does not identify documents that do not have the query terms, but have related words. For example, in a typical Boolean system, a search on “Australian Shepherds” would not return documents about other herding dogs such as Border Collies that do not have the exact query terms. Rather, such a system is likely to also retrieve and highly rank documents that are about Australia (and have nothing to do with dogs), and documents about “shepherds” generally.
The problem here is that conventional systems index documents based on individual terms, rather than on concepts. Concepts are often expressed in phrases, such as “Australian Shepherd,” “President of the United States,” or “Sundance Film Festival”. At best, some prior systems will index documents with respect to a predetermined and very limited set of ‘known’ phrases, which are typically selected by a human operator. Indexing of phrases is typically avoided because of the perceived computational and memory requirements to identify all possible phrases of say three, four, or five or more words. For example, on the assumption that any five words could constitute a phrase, and a large corpus would have at least 200,000 unique terms, there would approximately 3.2×1026 possible phrases, clearly more than any existing system could store in memory or otherwise programmatically manipulate. A further problem is that phrases continually enter and leave the lexicon in terms of their usage, much more frequently than new individual words are invented. New phrases are always being generated, from sources such technology, arts, world events, and law. Other phrases will decline in usage over time.
Some existing information retrieval systems attempt to provide retrieval of concepts by using co-occurrence patterns of individual words. In these systems a search on one word, such as “President” will also retrieve documents that have other words that frequently appear with “President”, such as “White” and “House.” While this approach may produce search results having documents that are conceptually related at the level of individual words, it does not typically capture topical relationships that inhere between co-occurring phrases.
Accordingly, there is a need for an information retrieval system and methodology that can comprehensively identify phrases in a large scale corpus, index documents according to phrases, search and rank documents in accordance with their phrases, and provide additional clustering and descriptive information about the documents.
Another problem with conventional information retrieval systems is that they can only index a relatively small portion of the documents available on the Internet. It is currently estimated that there are over 200 billion pages on the Internet today. However, even the best search engines index only 6 to 8 billion pages, thereby missing the majority of available pages. There are several reasons for the limited indexing capability of existing systems. Most significantly, typical systems rely on a variation of an inverted index that maintains for every term (as discussed above) a list of every page on which the term occurs, along with position information identifying the exact position of each occurrence of the term on the page. The combination of indexing individual terms and indexing positional information requires a very large storage system.
A further problem with many information retrieval systems used for searching the Internet is their inability to archive pages that change over time. Conventionally, most Internet search engines only store the relevance information for a current instance (or version) of a given page, and update this information each time the page is re-indexed. As a result, a given search only returns the current versions of pages that satisfy the query. Users are unable as a result to search for prior instances of pages, or pages that were current in a specific date interval. Also, the search engines likewise do not use version or date related relevance information when evaluating search queries or presenting search results.
Accordingly, it is desirable to provide an information retrieval system that can effectively index tens of billions, and eventually over 100 billion pages of content, without the substantial storage requirements of existing systems.